1. Field of the Invention
This invention relates to computer assisted surgery generally and more specifically to computer assisted total hip replacement (THR) or hip arthroplasty operations.
2. Description of the Related Art
Total hip replacement or arthroplasty operations have become increasingly common in the United States, with more than 300,000 such operations occurring annually. Many of the procedures will eventually require revision, due to one of any number of problems. Problems can arise with the implant, which can wear, degrade or even fracture. In other cases, dislocation of the replaced hip can occur, causing extreme pain (not to mention inconvenience and expense). The incidence of dislocation has remained at approximately 2-6 per cent, in spite of improvements to technique and materials.
It is known that the incidence of post-surgical dislocation is related to the orientation of the hip replacement components, particularly to the angular orientation of the acetabular shell component in relation to the bony anatomy. See Lewinnek et al., xe2x80x9cDislocation after total hip-replacement Arthroplasties,xe2x80x9d Journal of Bone and Joint Surgery, Vol. 60A, No. 2, PP. 217-220 (1978). The head and neck geometry of the implant is also thought to be a factor.
In spite of the published research, the typical surgeon has not adopted any sophisticated method of navigating hip replacement surgery, in spite of the availability of several techniques. The most prevalent method is to rely on an acetabular impactor tool with a handle placed at an angle predetermined so that if the handle is maintained at a level, horizontal orientation, the acetabular shell will be at a desired angle. This method fails to consider the considerable movement and variation in the patient""s pelvic position during surgery; at worst it aligns the shell with the operating table (not necessarily the pelvis). More technological methods have been developed, including the sophisticated method described in U.S. Pat. No. 6,205,411 (and related applications) to DiGioia et al. (2001). The method of DiGioia is an advance over the prior methods (which he summarizes authoritatively in his xe2x80x9cBackgroundxe2x80x9d section).
DiGioia""s method begins with extensive pre-operative imaging, including relatively expensive CT scanning. The pre-operative imagery is then input into a digital computer model, which performs extensive, three-dimensional modeling including range of motion simulations of the patient""s anatomy in relation to a specific computer model of a particular implant,. Next, in an intra-operative phase, the pre-operative models are registered with intra-operative optical tracking data: a very large number of points are sampled on the pelvis and femur, and the computer fits the data to the pre-operative model. Finally, the implant is positioned to align as closely as possible with the optimized computer model.
The method of DiGioia et al. is complex and requires sophisticated digital and radiological techniques. A need still exists for a simpler method of surgical navigation which will facilitate proper hip geometry with a minimum of pre-operative imagery and expense. It is frequently found that physicians are loath to adopt any methods, and particularly any computerized methods, which are unduly complex, expensive or time consuming. In this they may be forgiven, in light of the increasing economic constraints which burden the modern practice of medicine.
Thus, a need persists for an intra-operative computer assisted hip navigation system which is easily learned, rapidly executed, economically practical, and independent from expensive or exotic pre-operative radiological imagery.
In view of the above problems, the present invention includes a method and system for intra-operative navigation of a hip replacement operation which permits a surgeon to intra-operatively assess the orientation of an acetabular shell implant and/or the orientation of a femoral stem implant, without recourse to pre-operative imagery or computerized simulations.
According to one aspect, the invention is a computer assisted, non-radiological method of intra-operatively measuring and assessing relative geometric relationships among skeletal features of a hip joint, suitable for surgical navigation of a hip arthroplasty operation. The method includes the steps: defining a pelvic plane from at least three recognizable anatomic features of the pelvis; tracking with an optical tracking system the orientation of an acetabular implant, to obtain acetabular implant orientation data; and adjusting the acetabular implant into a desired orientation with respect to the defined pelvic plane, without reference to previously obtained radiological data, by relating the acetabular implant orientation data to the defined pelvic plane.
According to another aspect, the invention includes a device for tracking the upper femur, suitable for use during a hip arthroplasty operation. The device includes: a rigid collar which can engage the greater trochanter; an optical tracking target, mounted on the rigid collar; and a ligature, attached to the rigid collar and capable of being wrapped around the upper femur and tensioned to urge the collar against the greater trochanter, for attaching the rigid collar to the femur.
Still another aspect of the invention is a method of attaching a tracking marker to the upper femur, suitable for tracking the upper femur during hip arthroplasty or a similar operation, including the steps: positioning a rigid collar in contact with the greater trochanter of the femur, the collar adapted for mounting thereon a tracking marker; attaching a ligature or opposable clamp to the collar; wrapping the ligature around the upper femur; and tightening the ligature or clamp around the femur to pull the collar tightly against the greater trochanter.
Another aspect of the invention is a method of determining changes between (1) pre-operative femoral position and (2) post-operative implant geometry, suitable for use during a hip arthroplasty operation, including the steps of: Maneuvering the femur into a reference position; measuring, with a non-radiological optical tracking device, pre-replacement femoral parameters; after implanting a prosthetic, returning the femur to the reference position; again measuring, with a non-radiological optical tracking device, post-replacement femoral parameters; and comparing the pre-replacement and the post-replacement parameters in a computer model.
The system of the invention includes: a locating system; a computer, interfaced to the locating system and interpreting the positions of tracked objects in a generic computer model of a patient""s hip geometry; a software module, executable on the computer, which defines the patient""s pelvic plane without reference to previously obtained radiological data, by locating at least three pelvic landmarks; and a pelvic tracking marker, fixable to the pelvic bone and trackable by the locating system, to track in real time the orientation of the defined pelvic plane.
Preferably, the system also includes a femoral tracking marker, securely attachable to a femur of the patient by a non-penetrating ligature and trackable by the locating system to detect changes in leg length and femoral offset.
Preferably, a system in accordance with the invention also includes a method of verifying reliability of a computer assisted, optically tracked surgical navigation system. According to this method, a fixed optical tracking marker is tracked in relation to a fixed reference mark on the same bone, both before and after the surgical procedure. This provides a check (xe2x80x9ctracker checkxe2x80x9d) to detect any errors due to, for example, slippage, drift, or deformation of the apparatus.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: